Optical comparators, particularly of a type that project a shadow image (e.g., a negative image) or an obliquely illuminated image of a test part onto a screen overlaid by a template of the test part, have enjoyed widespread acceptance as robust and reliable forms of measurement by providing measurement results in a visually verifiable form. Differences between projected edge features of the test part and one or more tolerance boundaries of the same features inscribed on the templates are readily apparent on the comparator screens. While numerical data can also be extracted by monitoring motions of the test parts against calibrated images of the test parts appearing on the comparator screens, the numerical data can be verified for reasonableness against estimates made by visually inspecting the edges of the test part against the template boundaries or other edge features of at least approximately known size appearing on the screens.
The accuracy with which visual comparisons can be made depends largely on the accuracy with which the templates can be made. The optics of the optical comparators can be carefully calibrated and optically corrected to present largely distortion-free images of test parts mounted within the viewing apertures of the optical projection systems. However, different templates can be required for different test parts or more complex templates in the form of chart gages can be constructed for measuring ranges of related edge features among corresponding sets of test parts. More than one template can be required for measuring multiple views or edge features of the same test parts. The templates can be expensive to manufacture to required accuracy and can require special storage and handling provisions to preserve the templates in working condition. Time must be allotted for ordering new or replacement templates, and even minor revisions to the intended form or tolerance definitions of test parts can require the ordering of new templates.
Efforts have been made to replace optical projection comparators with digital camera based comparators for making similar visual comparisons on computer monitors. Most advantageously, physical templates can be replaced by digital representations of the test parts generated on the computer monitors by extracting boundary information from computer-aided design (CAD) specifications that define the intended outlines of the test parts. Digital images of the test parts captured by digital cameras are also generated on the same computer monitors for making comparisons against the digitally generated templates. However, the computer monitors present pixilated images that significantly limit the precision with which the comparisons can be made for given size images appearing on the computer screen. Since both the templates and the test parts must be matched to the same scale on the computer screens, the resolution of the templates and test parts match each other at different digital magnifications. Thus, fine comparisons can be difficult to make to customary certainty and can require visual inspection of smaller segments of the test parts within the same size field of view. The digital conversion of the test part images contributes additional systematic and random errors that reduce the reliability of the measurements as well as the perceived robustness previously derived from comparing actual images of test parts against reference datum.